The HIV envelope glycoprotein complex (gp120/gp41) mediates viral entry by binding to host cell receptors and promoting fusion of the virus and cell membranes. A major challenge in producing an envelope glycoprotein immunogen capable of eliciting a potent primary virus neutralizing antibody response is thought to lie in preserving the correctly folded conformations of the complex. Central to the stability and functioning of the envelope glycoprotein is the interface between the gp120 and gp41 subunits. These molecular interactions retain the metastable form of the gp41 molecule on the free virion and mediate the activation of gp41 upon gp120 binding to CD4 and coreceptor. Our research objective is to develop a working model of the molecular determinants that control the gp120-gp41 interface. We have recently identified mutations within the six helix core of gp41 that specifically affect the gp120-gp41 association. These data add to previous findings that positions within the extended N- and C-chains and the disulfidebonded loop region are important for gp120-gp41 association, and lead us to hypothesize that the larger central ectodomain of gp41 may be critically involved in the stability and functioning of the resting prefusogenic envelope glycoprotein. In this proposal, we seek: (1) To determine, by scanning mutagenesis, key amino acid side-chains within the central ectodomain of gp41 that specifically affect the gp120-gp41 interface. (2) To examine the basis for the mutant phenotype by using specific inhibitory agents and neutralizing monoclonal antibodies to probe the interactions between the effect of the mutation and the mechanism of inhibition or neutralization. (3) To isolate second-site revertants in gp120 that complement the fusion deficiency engendered by the mutation in gp41. These studies will yield a functional map of the interactions between the gp120 and gp41 subunits, and will provide a unique perspective towards understanding the structure-function relationships of the native envelope glycoprotein complex. This knowledge will guide efforts to design envelope glycoprotein immunogens and to develop antiviral inhibitors to prevent viral entry. [unreadable] [unreadable]